Although it seems like volumes have been writtenrecently about the ubiquitous plastic pollution in the world’s waterways, a new report finds major gaps in where we’ve looked and what we know.

Few, if any, studies have been done in critical regions like the waters off populous coastal Asia, according to the research overview published online today in the journal Environmental Reviews. The paper also shows that most of the focus has been on oceans, with food web impacts in lakes and other freshwater ecosystems largely unknown.

Studies on the Same Few Regions

Little is known about how up to 12.7 million metric tons of plastics entering the ocean every year move through aquatic food webs.Plastics pollution is currently “a hot topic,” said lead author Jennifer Provencher, head of the Wildlife Health Unit for the Canadian Wildlife Service’s Wildlife Management and Regulatory Affairs Division, based in Gatineau, Quebec. But despite a spate of recent studies, relatively little is known about how up to 12.7 million metric tons of plastics entering the ocean every year—and unknown amounts making their way into streams, rivers, and lakes—move through aquatic food webs.

In an effort to compile current knowledge and identify key research needs, Provencher and colleagues reviewed 160 recent field studies that reported plastic ingestion by animals. They found that most of the research so far has been concentrated in the same few geographic regions, ecosystems, and species.

“The regions that top out the studies are Newfoundland/Labrador and the Mediterranean,” said Provencher, “but then there are many marine regions where we have no data to date.”

Understudied regions include places like the South China Sea and the Gulf of Thailand, which are likely to be heavily affected by plastics pollution. Coral reefs and deep-sea habitats, areas known to be sinks for waterborne plastics, also have received relatively little research attention. In addition, only six papers examined freshwater ecosystems, Provencher said, “which shows a huge lack of understanding of how plastics may affect and be passed on by biota in these regions.”

The review included academic papers published through the end of 2016.

Plastics in Food Webs

Few of the reviewed studies examined ways that plastics could migrate through the food web, such as from prey to predator, known as trophic transfer. Only a handful looked for microplastic particles in animal tissues, which would provide evidence that plastics bioaccumulate, like many chemical pollutants do. And none investigated biomagnification, or whether plastics grow more concentrated in animals higher up the food chain, similar to what happens with polychlorinated biphenyls (PCBs) and other harmful chemicals.

The paper points to some critical needs, including more research on plastics in the freshwater food web.The review of field studies “highlights quite a few critical data gaps in our understanding of trophic transfer in particular,” said Mark Hahn, a toxicologist and senior scientist in the Biology Department at the Woods Hole Oceanographic Institution in Massachusetts, who studies plastics but was not involved in the current research.

Given that many facets are underresearched, the paper points to some critical needs: more research on plastics in the freshwater food web, more studies over a broader range of ocean locations, and more surveys on multiple species at each site to see whether and how the contaminants are passed along.

A Chemical Contaminant?

With plastics piling up seemingly everywhere and the tiny fragments being found in food and water now turning up in human and seabird feces, scientists say tracking their pathway across the aquatic environment is vital to addressing the problem.

“Can plastics move through the food chain like chemical contaminants?”Plastics in human feces “should come as zero surprise to everybody,” said Mark Mallory, a biologist and professor at Acadia University in Wolfville, Nova Scotia, and coauthor on the new paper. “The big question is, Can plastics move through the food chain like chemical contaminants?”

To control plastic pollution, “we need to know where is the problem biggest, how is it moving in the environment,” Mallory said. He added that scientists may need to start taking a “whole ecosystem approach to plastics just like we would with chemical contaminants.”

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